The present invention relates to fluorinated powder coating products, and to a process for coating galvanized steel with a fluorinated powder coating. In particular, the present invention relates to the use of vinylidene fluoride copolymers in pigmented powder coating products, for obtaining corrosion-resistant PVdFC-based protective coatings on galvanized steel.
Coatings based on vinylidene fluoride polymers (hereinafter PVdF) are known to be very useful as protective coatings for a variety of substrates due to the good chemical and weathering resistance and to the thermal stability of PVdF. The general known technique for preparing them is to prepare a dispersion of PVdF in a suitable solvent for application by known means onto the required substrate which is thereafter subjected to a heat treatment.
The solvent used is generally known in the prior art as "latent solvent", and it is described therein as an organic solvent which has no significant action on PVdF at room
However, although the known systems may give good results, environmental protection laws throughout the world, but mainly in Europe and in the U.S.A., are making it increasingly difficult to handle solvent-based systems. Further, the recovery of the solvent is a costly procedure. Accordingly, there is a need in the art for solventless PVdF-based coatings.
British Patent GB 2194539-A discloses pigmented PVdF-based powder coating products essentially consisting of vinylidene fluoride homopolymer (or copolymer with at most 10 wt % of comonomer units), one or more compatible thermoplastic resins and one or more pigments. It also discloses a process for preparing the products. However, the required baking temperature is unacceptably high.
European Patent EP-456018-A discloses pigmented powder coatings of vinylidene fluoride/hexafluoropropene copolymers, the resin component comprising 50-90 wt % of said copolymers having a melt viscosity of 1-4 kPoise at 100.sup.-1 and 232.degree. C., said copolymers being characterized by a melting point in the range of about 160.degree. C. to about 170.degree. C. and 50-100 wt % of thermoplastic acrylic resin. The resulting coatings are said to be characterized by improved flexibility, crack resistance and surface smoothness (as opposed to surface roughness or "orange peel" associated with the finish) while not requiring the inclusion of a flow-improving agent. However, the requisite baking temperature is also unacceptably high for use on galvanized steel.
Indeed, galvanized steel should not be heated above 200.degree. C. (J. Van Eijnsbergen, Galvano-Organo 61(629)775-8, 1992), and the higher temperatures required to bake PVdF-based powder coatings has up to now prevented their use on galvanized steel. There is thus a need in the art for lower temperature application, fluorinated powder coatings for galvanized steel, said coatings having good anticorrosive properties.
It is an object of the invention to provide low temperature fluorinated powder coatings for galvanized steel.
Another object of the invention is to provide low temperature fluorinated powder coatings for galvanized steel, said coatings having good anticorrosive properties.
A further object of the invention is to provide low temperature powder coatings for galvanized steel, said coatings having high gloss.
Yet another object of the invention is to provide a process for coating galvanized steel with a fluorinated powder coating.
Accordingly, the present invention provides a pigmented PVdF-based powder coating product for galvanized steel, essentially consisting of:
(a) a resin component essentially consisting of: PA1 (b) from 1 to 35 parts by weight of one or more pigments per 100 parts by weight of the resin component. PA1 (i) a resin component essentially consisting of: PA1 (ii) from 1 to 35 parts by weight of one or more pigments per 100 parts by weight of the resin component. PA1 (i) melt-mixing the VdFC, the compatible components and the pigments; PA1 (ii) forming pellets; and, PA1 (iii) grinding the pellets.
(i) from 60 to 90 wt % of one or more vinylidene fluoride copolymers having a melting temperature below about 160.degree. C.; PA2 (ii) from 40 to 10 wt % of one or more compatible resins; and, PA2 (a) from 60 to 90 wt % of one or more vinylidene fluoride copolymers having a melting temperature below about 160.degree. C.; PA2 (b) from 40 to 10 wt % of one or more compatible resins; and,